Comparing detectability of marked and unmarked individuals

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Comparing detectability of marked and unmarked individuals

Postby creemts » Tue Mar 18, 2014 3:15 pm

All,
I am working with a small species of cactus. We have permanent plots that we visited 7 times last year (visitation intervals varied among the plots). During each visit, we searched the plot for cacti and placed tags next to all the cacti we found; we also noted whether previously tagged cacti were alive, flowering, fruiting, dead or (rarely) missing. In most cases, we doubled the number of cacti tagged in the plot between the first and last visit. While a small proportion of that increase was reproduction, most of the new cacti found at each visit were too large to be seedlings. Overall, I have around 1400 tagged individuals, with plots having anywhere from 30 to 500 cacti.

I want to calculate detection probability for both tagged and untagged cacti. Based on plant size, I can identify cacti that were present at the first visit, but were not detected until later visits. I have been working my way through the Gentle Introduction to MARK and have calculated the resighting probability for the tagged plants (around 99%), but I'm puzzled by how to analyze the untagged cacti. Any suggestions for analyses, programs, references (including sections in the MARK book I may have missed) would be greatly appreciated. Ideally, I would like to include cactus size, height, flowering status and fruiting status (at the first detection) as covariates, since my field experience suggests that all of these factors are important. This is my first detection probability analysis, so even basic advice would be greatly appreciated.

Charlotte
creemts
 
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